1 Answer
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You need to have editable keyboard accelerators turned on as documented under "How do I assign different keyboard shortcuts?" on this page*. Then hover over menu Go->Open Parent and press backspace twice.

That should add this line:

(gtk_accel_path "<Actions>/ThunarWindow/open-parent" "BackSpace")

to your ~/.config/Thunar/accels.scm

*This setting is also exposed in the Gnome Configuration Editor under...